1. Field of the Invention
The invention generally relates to a frequency error correction for use in units or subunits of communication systems, and in particular for use in WLAN (Wireless Local Area Network) receivers.
2. Description of the Related Art
In a communication system, it is important for a receiver to synchronize to the transmitter so that messages can successfully be exchanged between the transmitter and the receiver. In a radio communication system, it is particularly important that a receiver is tuned to the frequency of the transmitter for optimal reception.
A wireless local area network is a flexible data communication system implemented as an extension to or as an alternative for a wired local area network (LAN). WLAN systems transmit and receive data over the air using radio frequency or infrared technology to minimize the need for wired connections. Thus, WLAN systems combine data connectivity with user mobility.
Most WLAN systems use spread spectrum technology, a wide-band radio frequency technique developed for use in reliable and secure communication systems. The spread spectrum technology is designed to trade-off band-width efficiency for reliability, integrity and security. Two types of spread spectrum radio systems are frequently used: frequency hopping and direct sequence systems.
The standard defining and governing of wireless local area networks that operate in the 2.4 GHz spectrum is the IEEE 802.11 standard. To allow higher data rate transmissions, the standard was extended to the 802.11b standard, that allows data rates of 5.5 and 11 Mbps in the 2.4 GHz spectrum. This extension is backwards compatible.
The standards for WLAN systems using direct sequence spread spectrum techniques employ a training preamble to train the receiver to the transmitter. Each transmitted data message comprises an initial training preamble followed by a data field. The preamble includes a sync field to ensure that the receiver can perform the necessary operations for synchronization. For the preamble length, two options have been defined, namely a long and a short preamble. All compliant 802.11b systems have to support the long preamble. The short preamble option is provided in the standard to improve the efficiency of the networks throughput when transmitting special data such as voice and video. The synchronization field of a preamble consists of 128 bits for a long preamble and 56 bits for a short preamble.
A receiver detects the synchronization symbols and aligns the receivers internal clock to the symbols in the synchronization field in order to establish a fixed reference timeframe with which to interpret the fields in the transmission frame structure following the preamble. The preamble, including the synchronization field, is transmitted at the start of every message (data packet).
In WLAN systems, as well as in other spread spectrum communication systems, the signal on its way from the transmitter to the receiver experiences several distortions. A frequency or phase error may result from a frequency or phase offset of the radio frequency oscillators at the transmitter and the receiver. The oscillators may further provide different frequencies due to manufacturing imperfections, different temperatures, etc. which result in a frequency drift off the baseband signal. It may be therefore the task of any synchronization unit within the receiver to perform an error correction.
Turning now to FIG. 1, a frequency synchronization of a conventional data communication receiver is schematically shown that comprises a frequency error correction 100 and a subsequent phase error correction 110. The frequency error correction 100 is performed to compensate for the frequency difference, and the phase error correction 110 will then compensate for the residual phase error. Thus, the phase error corrector 110 has the task to remove the phase-offset remaining on the data-path signal, such that a coherent reception is enabled. This minimizes the probability of demodulation errors.
Frequency error correction units in receivers still have a number of disadvantages. One problem is that frequency error correction units need to perform a time-consuming number of iterated steps to achieve a frequency synchronization. If a frequency error correction procedure is repeatedly performed in a feedback loop, a first frequency error correction, therefore, is only possible after completing the feedback loop.
Another problem may be that the conventional adjustment processes may sometimes not be performed with sufficient phase or frequency resolution, and are restricted in use by the individual capabilities of the respective hardware implementation.